Electrolytic process for removing oxide from the surface of metals



Patented Jan. 16, 1951 1 ELECTROLYTIC PROCESS FOR- REMOVING OXIDE FROM THE SURFACE OF METALS 4 Harry R. Spence and Horace W. Hooker, Lewiston,

N. Y., assignors to Hooker Electrochemical Company, Niagara Falls, N. Y., a corporation of New York No Drawing. Application December 29, 1945, Serial No. 638,448

.This application is a continuation-in-part of our co-pending application, Serial No. 538,538, filed June 2, 1944, now Patent No. 2,395,694, reissued as Re 22,887 on June 3, 1947.

Our invention relates more particularly to processes for removing oxide from the surface of metals, and especially the ferrous metals, also copper, and their alloys, which have been or are in the process of being rolled out into sheets or drawn into tubes, Wires or other shapes, at more or less elevated temperature. This work generally involves heating the metal and passing it several times through the rolls at temperatures sufliciently elevated to soften the metal, At these temperatures metal picks 'up oxygen from the air and forms the dark oxide. Ordinarily, this oxide readily scales off; but in the course of the working above described, the metal becomes toughened and hardened and the oxide film rolled into it until, in the case of the ferrous metals and their alloys, it may form a dense, coal black glaze. There may also be several finishing passes through dies at lower temperatures, such as result 7 Claims. (Cl. 204-145) 2 may form a sponge layer on the surface of the metal, or it may fall off into the electrolyte, where it accumulates a sludge. This process is open to the objection that it is very diflicult to secure uniform distribution of the current over the surface of the metal because of electrical resistance at the contacts, and especially through the oxide, and unequal distances from the anode of the var- I ious parts of the metal pieces undergoing treatment. This is particularly true when the metal is in the form of shapedarticles, such as stampings, coils of wire or tubes having recesses or interior surfaces. The difliculty of current distribution, which is experienced in both cathodic as well as anodic electrolysis, is especially great in k the case of the alloy steels, particularly those from the work of friction and deformation. For

this purpose it is necessary to remove the glaze and at intervals also to anneal the metal, in order to proceed. Removal of the glaze is difficult, especially in the case of the alloy steels. Annealing after removal of the glaze reoxidizes the metal.

Among the ferrous metals that maybe treated by our process, besides iron, are the other metals of the ferrous group, including nickel and cobalt, and their alloys with each other and withchromium, manganese, molybdenum, tungsten, vanadium and copper, including the stainless steels containing nickel and'chromium and alloys of nickel with chromium, such as Nichrome and Tophet, which may contain little or no iron, also alloys of nickel with copper, such as Monel, which contains no iron.

Two methods in general have been used or proposed for removing oxide from the surface of such metals and alloys, namely:

(a) The oxide may be removed by the purely chemical action of an aqueous acid, converting it into a soluble salt. This leaves the surface bright, but as the metal is also attacked by the acid, there is an appreciable loss of metal and the surface is left pitted. Moreover, the time required for the operation is excessive, e. g., several hours.

(b) The oxide may be reduced to the metallic state. This is done by cathodic electrolysis,,gen-

theumetal serves ascathode. The reduced oxide containing nickel or chromium or both, owing to high electrical resistance of these alloys and variations in their oxides. Non-uniform current distribution tends to localize the electrolytic action.

On this account it has sometimes been found advantageous to subject the metal to a brief anodic treatment in a bath of fused caustic alkali,- prior to the cathodic treatment. A typical treatment of this nature would be an anodic oxidation of 30 seconds followed by a cathodic reduction of 2 minutes, in a bath of fused caustic soda, at 500 to 550 C. and a current density of to 250 amperes per square foot of surface of the metal under treatment. In the case of stainless steel, and in particular that containing 18 per cent chromium and 8 per cent nickel, known as 18-8 stainless steel, this treatment leaves the surface in a rough condition, due to adherence of the, metal left b reduction of the oxide.

Such a surface is objectionable from the point of view of appearance and also in that it renders further rolling or drawing difficult. In order to remove this rough coating it is simetimes necessary to immerse the metal in a strong acid, such that it may be removed by a wash and quick dip in a dilute non-oxidizing acid, without any injury. to the metal surface. somewhat longer time cycle than the process employing cathodic reduction, it is simpler and elimeinates the harmful drastic acid treatment.

Although this involves a We have also found that an effect comparable with that of the anodic oxidation may be secured by subjecting the metal to the action of an an-- hydrous oxidizing agent that is effective without electrolytic decomposition, such as an alkali metal or alkaline earth nitrate, chlorate, peroxide, dichromate, manganate, or permanganate, without any electrolysis. The oxidizing agent may be added to the fused bath. Thus oxidation and caustic soaking steps become simultaneous and a single operation, and the process becomes nonelectrolytic. However, the time cycle is shortened if anodic oxidation is applied while the metal is in the fused bath.

The non-electrolytic embodiment of our process 1 is claimed in our reissue patent above referred to. It consists in soaking the article in a bath of fused caustic alkali containing 1 to 20 per cent, and preferably 5 to 16 per cent, of an active oxidizing agent, at a temperature between 36%" and 609 C. and preferably at about 500 C., depending upon the oxidizing agent, for l to 29 minutes, the time depending on the caustic alkali. and oxidizing agent used and upon the proportion of oxidizing agent in the bath, and then dipping it briefly, e. g.

for to 60 seconds, in a dilute non-oxidizing acid, such as l to 28 per cent hydochloric acid, preferably at 65 to 85 C. It is desirable to wash off excess caustic alkali before the acid dip. A convenient way of doing this is to quench the metal in water as it comes from the fused bath. When this is done, much of the oxide is found in the water; also there is less consumption of acid. The electrolytic treatment, with all its diiiiculties of current distribution, is thereby completely eliminated.

However, when the metal to be treated is in a simple form'which presents no difficulty in the way of uniform current distribution, anodic elec trolysis while the metal is in the fused caustic bath, without any subsequent cathodic electrolysis, but with a final dip in weal: non-oxidizing acid, may be distinctly advantageous. Examples of such metals are elongated rods or wires, such as are usually marketed in coils. For the purpose of thetreatment the straight rod or wire is passed through the bath in the direction of length, with a continuous movement. In this case, anodic oxidation serves to shorten the time cycle or, in other words, to increase the speed of passage in the bath. According to the electrolytic embodiment of our present invention, therefore, anodic' oxidation is applied to the metal, at least during the initial stage of its treatment in the fused caustic bath, preferably at a current density of 100 to 250 amperes per square foot of surface. The composition of the fused caustic bath is similar to that in the non-electrolytic embodiment of our process, i. e. the oxidizing agent is selected from the group consisting of the alkali metal and alkaline earth chlorates, nitrates, dichromates, manganates and permanganates. The bath is maintained at preferably 300 to 609 C.

It should be noted that this process is entirely difierent from those of the prior art, above described, in which there has always been a final cathodic reduction, and that in our process any cathodic reduction tends to undo the effect of the oxidizing agent upon the metal.

Our treatment changes the physical character of the'oxide so that the glaze is puffed out into a coating of fine grain, which may resemble lamp black. In some cases, however, the coating is brown and if the treatment is continued the brown oxide evenually becomes so puffed out and loosened that some of it may be shaken off. The resulting powder is of the fineness of paint pigment. i'he extreme fineness of this oxide probably accounts for the readiness with which it is attacked by the dilute acid. The removal of this oxide by the acid dip leavesa 'frosted' or mat surface, of the kind that is preferred for further drawing, and of a color from steel grey to silvery white, depending on the nature of the steel.

When the metal is stainless steel, we have found that after use the bath contains sodium chromate in solution. ihis might be thought to indicate that the chromium is oxidized, the oxide forming with the caustic soda the sodium salt of chromium which then dissolves off in the bath.

Our fused alkali bath may consist of caustic soda, caustic potash or a mixture of these alkalies with each other or with their carbonates or caustic lime. The presence of the oxidizing agent lowers the melting point of the bath. Thus, 10 per cent" of sodium nitrate lowers the melting point of caustic potash from 360 to 315 C. Also, by proper proportioning of a mixture ofalkalies, it is possible to produce a bath Of'Sllb" stantially lower melting point than that ofany one of its individual components: Thu amix=-- ture of caustic soda and'caustic potash in equ'al proportions by weight melts at 205 (3., whereas these alkalies, in commercial grades, meltat 325 and 350 C., respectively. Such a" low melting bath will have in higher degree the property of wetting or soaking into the oxide and byusing-on'e of the more active oxidizing agents, such as tlie alkali metal and alkaline earth'chlor'ates, may be operated at lower temperaturethana bath-of-a single caustic alkali. Naturally the fav0ra-blep1'o'-' portions for such mixtures include their "eutectics;

Our process therefore effects not only a great simplification and substantial ch'eapenin'g of the operation, as compared withearlier processesin volving one or more electrolytictreatments in cluding a final cathodicreduction followed by a drastic treatment'with strongacid; butalso a sub stantial improvement in the result.

Example I Example I I Another piece of the same rod'was immersed for 25 minutes in a fused bath of caustic soda and caustic potash in equal proportions by weight containing 10 per cent sodiumch-lorate at 300 C. It came out with the oxide converted we coating resembling reddish brown paintpigment'i A water wash and acid dip of one minute removed the brown coating, leaving a clean frosted surface,

EscampleIII A strip of rolled low carbon steel having-aglos'sy" black surface was immersed for l0 minutes ma bath of fused caustic soda containingfi /2 per ce'nt' sodium nitrate by weight at about 409*(31- and withdrawn. The oxide has been converted the condition resembling paint pigment; so intensely black that it made the original surface seem-grey by comparison. A water washand acid d ip re- I movedthe'coating, leaving the surface a natural steel grey color.

Emample IV A strip of Nichrome containing 80 per cent nickel and 20 per cent chromium and no iron, heavily coated with dark oxide, was immersed in the same bath as in Example I for the same time and at the same temperature. It came out with a reddish brown irridescent coating which was completely removed by the Water and acid dip, leaving a beautiful shiny surface.

Example V A sheet of Monel coated with dark oxide was immersed in a fusedbath of caustic soda containing 6 per cent sodium nitrate at 500 C. for 5 minutes. It came out with the oxide converted to a coating resembling lamp black. It was quenched in water and immersed in warm per cent hydrochloric acid forone minute. The surface was left a beautiful silvery white.

Example VI alloy steels, that is only because they present the greatest difficulty. A large field of application is of course that of ordinar low or high carbon steels. It is to be understood that our process is also applicable to alloys containing copper and, in general, to any metal or alloy that is resistant to caustic alkalies under the conditions of treatment, and preferably up to at least 300 C. to 600 C.

Although in the foregoing specification and examples we have indicated that the acid should be non-oxidizing and have used only dilute hydrochloric acid as an illustration of such an acid, we do not wish to be limited thereto. Other acids, including nitric and sulphuric acid, which are ordinarily considered as oxidizing acids, may be used if in such dilution and at such a temperature that they do not have an oxidizing effect or otherwise attack the metal.

While we do not wish to be held to any particular theory as to the reactions that take place in our process, it would seem that the fused caustic alkalies form the correspondin salts of the metal oxides which are generally black but sometimes a very distinctive reddish brown, and that these salts formed in the molten bath being basic as well as finely divided, are consequently very reactive with acids. It appears that the presence of the oxidizing agent promotes formation of these reactive salts and any preliminary reduction of the original metal oxides, as by cathodic electrolysis, hinders their formation. The presence of any considerable quantity of water would, of course prevent the necessary temperature from being reached.

We claim as our invention:

1. The method of removing from the surface 6 1 ing of a caustic alkali containing 1 to per cent of an oxidizing agent of the group consisting of the alkali metal and alkaline earth chlorates, nitrates, dichromates, manganates and permanganates; simultaneously passing electric current from the metal connected as anode through the fused bath; and then subjecting .the a I unreduced modified oxide to the action of a dimanganates and per-manganates; simultaneously passing electric current from the metal connected as anode through the fused bath; and

then subjecting the unreduced modified oxide.

to the action of a dilute inorganic acid.

' 3. The method of removing from the surface of metals of the group consisting of iron, nickel and cobalt and their alloys with each other and with chromium, manganese, molybdenum, tungsten, vanadium-and copper dense firmly adherent oxide that has formed thereon through exposure to air under oxidizing conditions, which comprises: immersing the metal in a fused substantially anhydrous bath consisting of caustic alkali containing 1 to 20 per cent of an oxidizing of metals resistant to fused caustic alkalies dense firmly adherent oxide that has formed thereon through exposure to air under oxidizing conditions, which comprises: immersing the surface in a fused substantially anhydrous bath consistagent of the group consisting of the alkali metal and alkaline earth chlorates, nitrates, dichromates, manganates and permanganates, at 300 to 600 0.; simultaneously passing electric current through the metal as anode for at least part of the time; and then subjecting the unreduced modified oxide to the action of a dilute inorganic acid.

4. The method of removing from the surface of metals resistant to fused caustic alkalies dense firmly adherent oxide that has formed thereon through exposure to air under oxidizing conditions, which comprises: immersing the metal in a fused substantially anhydrous bath consisting of caustic alkali contianing l to 20 percent of an oxidizing agent. of the group consisting of the alkali metal and the alkaline earth chlorates, nitrates, dichromates, manganates and permanganates, at 300 to 600 0.; simultaneously passing electric current through the metal as anode for at least a part of the time, at a current density of to 250 amperes per square foot; and then subjecting the unreduced modified oxide to the action of a dilute inorganic acid.

5. The method of removing from the surface of metals resistant to fused caustic alkalies dense firmly adherent oxide that has formed thereon through exposure to air under oxidizing conditions, which comprises: immersing the metal in a fused substantially anhydrous bath consisting of a caustic alkali containing 1 to 20 per cent of an oxidizing agent of the group consisting of the alkali metal and alkaline earth, chlorates, nitrates, dichromates, manganates and permanganates, at 300 to 600 0.; simultaneously passing electric current through the metal as anode for a part of the time; discontinuing the electric current and causing th metal to soak in the bath for a further period; and then subjecting theunreducedmodified oxide to the action of a zdilute inorganic acid.

6. The method of removing from the surface 'ofsmetal rods and wires resistant to'fused caustic alkalies dense firmly adherent oX-idethat has formed thereon through-exposure to air under oxidizing conditions, which comprises: passing the metal longitudinally through a, fused substantially anhydrous bath consistingof'caustic =a1kali containing 1 to 20per cent of an voxidizing agent of thegroup consisting of the alkali metal and alkaline earth, chlorates, nitrates, dichromates, manganates andtpermanganates, at.300 to 600 0.; simultaneously passing electric currentthrough the metal asanode; and then passing the metal longitudinally through a bath of dilute inorganic acid.

'7. The method of removing from thesurface of metals resistant to fused caustic-alkalies, dense firmly adherent oxide thatrhas formed thereon through exposure to air under oxidizing conditions, Which comprises: immersing the metalin a .fused substantially anhydrous bath consisting of caustic soda containing llto 20 per cent of sodium nitrate, at 300 to 600 C.;-- simultaneously passing electric current through the metaluas anode; and then subjecting the unredueed modified oxide to the action of a dilute inorganic acid.

HARRY R. SPENCE. HORACE W. HOOKER.

REFERENCES CITED The following references are of record in the 'file of this patent:

UNITED STATES PATENTS OTHER vRhFFRENCES p s f pplied Electrochemistry, by Allmand, 2d ed., published in 1924, pages 500, 501, 

1. THE METHOD OF REMOVING FROM THE SURFACE OF METALS RESISTANT TO FUSED CAUSTIC ALKALIES DENSE FIRMLY ADHERENT OXIDE THAT HAS FORMED THEREON THROUGH EXPOSURE TO AIR UNDER OXIDIZING CONDITIONS, WHICH COMPRISES: IMMERSING THE SURFACE IN A FUSED SUBSTANTIALLY ANHYDROUS BATH CONSISTING OF A CAUSTIC ALKALI CONTAINING 1 TO 20 PER CENT OF AN OXIDIZING AGENT OF THE GROUP CONSISTING OF THE ALKALI METAL AND ALKALINE EARTH CHLORATES, NITRATES, DICHROMATES, MANGANATES AND PERMANGANATES; SIMULTANEOUSLY PASSING ELECTRIC CURRENT FROM THE METAL CONNECTED AS ANODE THROUGH THE FUSED BATH; AND THEN SUBJECTING THE UNREDUCED MODIFIED OXIDE TO THE ACTION OF A DILUTE INORGANIC ACID. 